state the condition into which the conducting wire is thrown before
it is moved, we shall then be in a position to understand the
physical constitution of the electric current generated by its
motion.

In this inquiry Faraday worked with steel magnets, the force of
which varies with the distance from the magnet. He then sought a
uniform field of magnetic force, and found it in space as affected
by the magnetism of the earth. His next memoir, sent to the Royal
Society, December 31, 1851, is 'on the employment of the Induced
Magnetoelectro Current as a test and measure of magnetic forces.'
He forms rectangles and rings, and by ingenious and simple devices
collects the opposed currents which are developed in them by
rotation across the terrestrial lines of magnetic force. He varies
the shapes of his rectangles while preserving their areas constant,
and finds that the constant area produces always the same amount of
current per revolution. The current depends solely on the number of
lines of force intersected, and when this number is kept constant
the current remains constant too. Thus the lines of magnetic force
are continually before his eyes, by their aid he colligates his
facts, and through the inspirations derived from them he vastly
expands the boundaries of our experimental knowledge. The beauty
and exactitude of the results of this investigation are
extraordinary. I cannot help thinking while I dwell upon them, that
this discovery of magneto-electricity is the greatest experimental
result ever obtained by an investigator. It is the Mont Blanc of
Faraday's own achievements. He always worked at great elevations,
but a higher than this he never subsequently attained.

Footnotes to Chapter 13